This invention relates to an electrochromic device of a board form which is for controlling transmittance of light therethrough. More particularly, the device herein concerned with is a board produced by superposition of two electrochromic cells each of which utilizes a combination of an electrochromic material that takes on color in its electrochemically oxidized state and another electrochromic material that takes on color in its electrochemically reduced state.
It is known to use an electrochromic (EC) device in the form of a board as a device for controlling transmittance of light therethrough, such as a window pane or a filter for CRT or fluorescent displays.
Also it is known to use a combination of the aforementioned two types of EC materials in an EC device of board form having oppositely arranged two electrode layers. In operation of the EC device, electrochemical oxidation of the EC material on one electrode is accompanied by electrochemical reduction of the EC material on the opposite electrode. Accordingly simultaneous coloration of the two electrode layers and simultaneous bleaching of the two electrode layers take place. The primary purpose of this construction is intensifying coloration of the display device. For example, JP-A No. 59-159134 shows an EC display device of such a construction.
In the case of an EC device for controlling transmittance of light therethrough it is desired that the lowest level of transmittance which can be reached by coloration of the device be as low as possible. However, even when a combination of the aforementioned two types of EC materials is employed it was difficult to obtain an EC device that can reduce its transmittance of visible light to less than 7%. It is possible to further decrease transmittance of visible light by providing a colored filter to the EC device. However, when the EC device is bleached the filter becomes an obstacle to realization of a desirably high transmittance. This is very inconvenient particularly when the EC device is used as a window pane.
As a means to obtain a light transmittance controlling EC device which becomes sufficiently low in transmittance by coloration, we have proposed in Japanese Utility Model application No. 62-164908, filed Oct. 28, 1987 (not published yet), a double-decker device produced by superposition of two EC cells each of which uses a combination of an EC material that takes on color in oxidized state and another EC material that takes on color in reduced state.
In practice the double-decker structure offers an inconvenience in that the total number of leads increases. That is, the EC device must have at least four leads since the device includes four EC layers each formed on a transparent electrode layer spaced from the others. Furthermore, each EC cell in this device often includes an auxiliary electrode for use in an initial reduction treatment of one of the EC layers, and in such a case six leads extend from the EC device. The process of manufacturing the EC device includes sealing peripheral regions of each EC cell with an elastomeric sealant, and at that stage the large number of leads are liable to get entangled. Besides, an increased area of contact between the leads and the sealant causes degradation of airtightness of the EC device.